Laser diodes have been used in many prior art recording techniques as have monolithic laser diode arrays. Monolithic laser diode arrays used in recording typically contain 10-100 diodes and the recording is done with either photonic exposure or thermal exposure. Photonic systems react to the total exposure to photon energy, such that each photon striking the recording surface helps to expose it. Conversely thermal systems respond to peak temperatures and must reach a certain threshold for exposure to occur. Thermal systems usually operate in the infrared (IR), while photonic systems usually operate in the visible or ultraviolet (UV) range, but either system can operate in any range of the spectrum. Each diode may be a single mode source or a short multiple mode stripe and is said to record a particular "track" on the recording surface. Diode arrays can contain anywhere from 10 to 1000 diodes. In typical printing applications, the tracks on the recording surface are spaced between 10 and 20 microns apart, but for data storage applications, the tracks can be as close together as 0.5 microns, in order to permit high density recording.
A current problem associated with the use of monolithic laser diode arrays is the diode spacing within the array. Current technology in semiconductor fabrication can only produce arrays in which the diodes are spaced in the neighborhood of 10-100 microns and, as mentioned above, recording requires data spacing down to 0.5 microns. The laser diodes can not be de-magnified optically because of the large numerical aperture of the laser emission. Consequently, to achieve the required density of tracks on the recording surface, a non-optical method is required to reduce the effective diode spacing. Such methods normally include one of two techniques: angled diode arrays and interleaving.
An angled diode array is depicted in FIG. 1. The diode array 1 is maintained at an angle .theta. with respect to the perpendicular of the scan direction 11. Diode spacing a is typically between 10 and 100 .mu.m on the array, but because the array is angled, the spots 7 which are printed in the tracks on the recording surface 6 are more closely spaced with separations of b=a.cos .theta.. Printing the data onto the recording surface 6 in a linear fashion requires that the diodes of the angled array 1 be activated in a delayed fashion. The desired location of the printing dots 7 is in a horizontal line on the printing surface 6. Because the printing surface 6 is scanning (i.e. moving relative to the laser diode array 1) in direction 11, the various lasers must be delayed so that they are not activated until the desired location 7 on the printing surface 6 is reached. Diode 1a is not delayed, and data is fed straight into it. However, data flowing to diode 1b must be delayed slightly until spot 7b is directly under diode 1b. The required delay t is easily determined from the diode spacing a, the array angle .theta. and the scan velocity. The delay required for the other diodes 1c, and 1d is simply a multiple of that required for 1b. Using this technique of coupling the angled diode array with digital delays, the effective track spacing can be reduced on the recording surface overcoming the diode spacing limitation of semiconductor fabrication technology.
A second method of overcoming the diode spacing limitation requires interleaving. Interleaving involves multiple passes with a diode array, such that each pass fills in only a limited number of tracks and then subsequent passes fill in the remaining tracks in order to complete the recording. Both slanting and interleaving are well known and discussed in "High Power Multi-Channel Writing Heads", by Dan Gelbart, published in the "IS&T Tenth International Congress on Advances in Non-Impact Printing", Nov. 1994, which is hereby incorporated by reference.
A second major problem associated with monolithic diode arrays and their use in recording is the failure rate of the diodes. Moreover, if any of the diodes in the array fails, then the entire array is ruined and can no longer be used as a recording means. A need exists for a technique to overcome isolated failures of single diodes within the array, so that the array may still function.
Accordingly, it is an object of this invention to provide a fault tolerant diode array recording system which is capable of overcoming isolated diode failures within a diode array, so as to effectively record data onto a recording surface.